1. Field of the Invention
The present invention relates to a detachable top feed unit for sewing machines, for the type comprising: at least one top feed dog operatively connected to a rocking arm; a first control eccentric and a second control eccentric receiving a rotatory motion from an upper drive shaft, longitudinally housed in an arm of a sewing machine; a vertical-movement kinematic mechanism operatively interposed between the first eccentric and the rocking arm for transmitting a vertical reciprocating motion to the top feed dog; a horizontal-movement kinematic mechanism operatively interposed between the second eccentric and the rocking arm for transmitting a horizontal reciprocating motion to the top feed dog.
2. Prior Art
It is known that in sewing machines feed-advance movement of a workpiece while a sewing is being carried out usually relies on bottom feed dogs reciprocated across a so-called "needle plate" located flush with a cloth plate or work supporting surface along which the workpiece runs.
In order to be sure that the bottom feed dogs may act correctly on the workpiece for feeding purposes, a presser foot is provided that, supported by a presser bar, exerts a localized biasing spring action on the feed dogs.
The foregoing being stated, it is pointed out that under given situations it is convenient that the action of a top feed unit should be combined with the action exerted by the bottom feed dogs, which top feed unit is provided with reciprocating feed dogs acting upon the workpiece, at the presser foot too.
For example, the presence of the top feed unit is provided to be appropriate when the workpiece is made of a material offering a low smoothness degree under the presser foot.
As a further example, the top feed unit is adapted for use when two superposed fabrics must advance at different speeds while a sewing is being carried out for achieving particular puckering effects or others.
Presently, the top feed unit is integrally housed in a head which, as known, overhangs the work supporting surface and is part of the sewing machine.
The top feed unit substantially comprises a vertical-movement kinematic mechanism and a horizontal-movement kinematic mechanism both operated by respective eccentrics fitted to an upper drive shaft housed in the upper portion of the sewing machine and driven in rotation, as known, for causing operation, among other things, of a needle bar slidably guided through the head.
In greater detail, each of said kinematic mechanisms comprises a connecting rod operatively connected to the respective eccentric for transmitting, as a result of the rotation of said eccentric, an oscillating movement involving angular rotation to a corresponding intermediate shaft.
By suitable linkages, the oscillating movement of each intermediate shaft is converted to a horizontal or vertical reciprocating motion of a rocking arm to which the top feed dog is fastened. The combination of the vertical and horizontal reciprocating motions respectively transmitted by the above described kinematic mechanisms causes the top feed dog to move according to a substantially elliptical trajectory and suitably interfere with the workpiece for giving rise to the desired feed-advance of same.
After the above description it will be recognized that at the present time the presence of the top feed unit intails many restrictions in terms of flexibility of use of the sewing machine, referring in particular to the great number of situations in which the machine is to be used for carrying out workings in which the top feed unit is unnecessary or is even unacceptable.
In fact, being most of the components of the feed unit directly mounted within the machine head, the only solution to which an operator can resort for making the sewing machine adapted to the above workings in which the presence of said top feed unit is unacceptable consists in removing the feed dog from the corresponding rocking arm.
However, this expedient does not lead to satisfactory results in that the rocking arm reciprocating at the sewing area represents at all events an obstacle against the execution of certain particular workings, also because, in order to avoid risks of accidents, it is often placed within appropriate safety guards which are rather bulky.
In addition and above all, frictions and forces of inertia generated by the great number of components forming the top feed unit impair the operation speed of the sewing machine to a great extent, that is in the order of 30%.
In conclusion, due to the above restrictions the use of a sewing machine provided with the top feed unit appears to be inconvenient, above all with reference to industrial workings, when workings are to be carried out in which the presence of this feed unit is unnecessary. As a result, at the present time all workings for which the top feed unit is not required are executed on different sewing machines. Therefore in many cases sewing machines provided with said top feed unit are subjected to long periods of inactivity.
It is also to be pointed out that, at the present state of the art, the installation of the feed unit always involves the presence of a great number of actuating members in a very reduced space, that is the space available within the sewing machine head.
This situation is further made worse by the fact that the head must also necessarily contain other different members and/or kinematic mechanisms, such as for example the needle bar and the crank mechanism for its operation, the presser bar, and still other members.
Due to the above problems relating to bulkiness, it is always necessary to study and set up very complicated kinematic mechanisms involving high production costs and the installation of which is not convenient.
In addition said kinematic mechanisms just as a result of their complicated structure, are of uncertain reliability and involve many problems when servicing is needed. In the connection it is to be taken into due consideration the fact that any repair of even normal servicing operations to be carried out on the top feed unit inevitably bring about long periods of inactivity of the sewing machine.